This invention relates to a method for reducing the loss of water from stems and leaves of plants by transpiration and protecting plants from damage by wind and cold and to a composition for this purpose.
It has long been known that most of the water applied to agricultural crops in irrigation is lost by the process of transpiration, that is, the passage of water in the form of a vapor through the plant tissue. Thus, reduction of water loss by transpiration is important both because of widespread interest in decreasing requirements for irrigation water and, in certain types of environments, for alleviating water stress. Water stress appears as a decrease in plant water potential and turgor when the transpiration rate exceeds the water supply and the transpiration capacity of the plant. When water stress occurs, plant growth reduction is observed, even when the soil is moist.
It is thought that plants benefit from transpiration by improved mineral uptake and by cooling of leaves. However, it is believed that only a very low level of transpiration is required for mineral transport. Although complete cessation of transpiration might produce an increase in leaf temperature which could result in a detrimental, or even fatal, increase in respiration/photosynthesis ratio, it is generally believed that even a relatively large decrease in transpiration, up to about 40-50% is not detrimental to plants. See, generally, A. Poljakoff-Mayber et al., "Physiological Basis and Practical Problems of Reducing Transpiration," in "Water Deficits and Plant Growth," Volume III, New York, Academic Press, Inc., (1972), following 277, and D. C. Davenport et al, "Antitranspirants - Effects and Uses in Horticulture," "The American Horticultural Magazine," Volume 50, No. 3, after 110 (Summer, 1971).
As set forth in Poljakoff-Mayber, supra, four approaches to reduction of transpiration have been tried: (1) increasing leaf reflectance so as to reduce net energy absorption; (2) using windbreaks to increase air resistance to the transfer of water vapor; (3) enclosing plants to permit humidity buildup and decrease leaf to air vapor density gradient; and (4) applying materials which tend to close the stomata of leaves or which coat the leaf surfaces so as to produce a physical barrier to diffusion and enhance the resistance of the leaves to loss of water vapor.
The application of an inert material to leaves to prevent water loss was recorded as early as 300 B.C. by Theophrastus. More recently, Miller et al, investigated the use of wax emulsions for this purpose, "Studies on the development, preparation, properties and applications of wax emulsions, for coating nursery stock and other plant materials," Mich. Agr. Exp. Sta. Technical Bulletin Volume 218, 1-78 (1950).
Although it had erroneously been believed that filmforming polymeric materials such as polyethylene decrease transpiration because of higher permeability to carbon dioxide and oxygen than to water vapor, it has been found that all of the materials currently in use for this purpose are, in fact, appreciably more permeable to water than to carbon dioxide. Thus, the materials generally in use hinder photosynthesis and transpiration to approximately the same extent, possibly because the materials cover only parts of the leaves. Poljakoff-Mayber, supra, at 289.
Materials which have been used to reduce loss of water by transpiration include wax emulsions, as practiced by Cushman (U.S. Pat. No. 3,847,641), liquid polyterpenes (U.S. Pat. No. 3,676,102), polymers made from isocyanates as taught by Cooke (U.S. Pat. No. 3,539,373), long chain esters of lower organic acids as suggested by Gabor (U.S. Pat. No. 3,199,944) and soluble carboxylated polymers, for example, derived from Cellosolve.RTM. acrylate and methacrylic acid as taught by Ferguson (U.S. Pat. No. 3,157,964). Also, Klaas (U.S. Pat. No. 3,089,280) discloses the treatment of plants with an acrylic-based light-affecting composition containing optical brighteners. However, each of these compositions has one or more deficiencies. For example, the polymers used by Gabor are soluble, so that frequent applications are required to maintain effective protection against excessive transpiration. Although the main purpose of the Klaas coating is to produce a hard and brilliant finish on plants, the desired effect is achieved only by at least two coatings applied at intervals of about two to four weeks.
Polymer coatings have been applied to plants for other purposes, such as the coloring of grass. However, according to the teachings of Converse (U.S. Pat. No. 2,870,037) and Gardner (U.S. Pat. No. 2,786,821), the grasses so treated seem to have been protected from deterioration or crumbling through mechanical action of the polymer, rather than from loss of water by transpiration.
It is therefore apparent that although products are available which are essentially soluble and which are easily applied, such products are also washed from the sprayed plants following an unpredictable time interval based primarily on the frequency of rainfall. Thus, known compositions must be reapplied frequently to maintain their efficacy. Furthermore, most of the known products have varying stability to ultraviolet radiation in sunlight, so that the products deteriorate and then wash away. There is at present no product applied to the leaves and stems of plants which has an active applied life exceeding much more than about three weeks.
Therefore, there is a continuing need for transpiration-reducing and plant protecting compositions which are easily applied and have long-term efficacy and stability.